I have an airplane that has an ignition system that works like this. There are 2 energy producing devices, called magnetos that "fire" the spark plugs, 2 per cylinder. There is a 1 in a 1000 chance that one will fail.
What is the chance that Magneto A OR Magneto B will fail?
What is the chance that Magneto A AND Magneto B will fail?
What is the chance that Magneto A NOR Magneto B will fail?